A rating of a 4.0 means that 4 percent of all TV householRAB were watching the program. For network TV that total is about 114 million householRAB right, but it's less for cable TV obviously since not all people have cable, and that total nuraber changes a bit each year as more householRAB get televisions. I don't know what the conversion nuraber into cable ratings is though.
The second nuraber after that (for example, in a rating of 4.0/8, the 8) is a percentage of the total nuraber of individual people in TV householRAB, I believe. That total is about 286 million right now, but again it's less for cable TV. And it gets divided down into age groups. So the second nuraber would be the percentage of the total nuraber of people of the specified age group in a TV household watching the program, and then once more that nuraber is less for cable because there are householRAB with only broadcast television.
So basically the nurabers aren't straightforward at all. You'll have to do some nuraber crunching and reasearch to find real viewer nurabers.